The present invention relates to a relief valve. More particularly, the present invention relates to a thermally activated relief valve for use with a compressed gas storage cylinder (or pressure vessel).
In high pressure compressed natural gas systems, it is a requirement to provide a means by which the pressure vessel can be relieved of its gas charge in the event of an excessively high external temperature (e.g., a fire near the vessel). The standard approach has been to incorporate a fusible plug into the design of the system (pressure vessel, valve) that is continuously exposed to the direct pressure of the gas charge. A fusible plug is a fitting that contains a slug of eutectic material that blocks and seals an outlet passage while the external temperature is below a predetermined yield point. When the temperature of the fusible plug reaches the yield temperature, the fusible material melts to provide a pathway for the pressurized gas to escape.
In principle, this approach to temperature relief is acceptable. A problem arises, however, when the fusible plug is exposed to the continuous high pressures of the gas charge at temperatures approaching the yield point of the eutectic material. In such condition, extrusion of the fusible slug may occur, thus producing a potential leak path.
A number of varying strategies can be applied to correct this problem through modification of the fusible plug (e.g., reducing bore diameter, increasing the yield temperature of the eutectic material). None of the strategies mentioned have thus far completely eliminated the problem.